The ocean’s role in setting the mean position of the Inter-Tropical Convergence Zone

Abstract

Through study of observations and coupled climate simulations, it is argued that the mean position of the Inter-Tropical Convergence Zone (ITCZ) north of the equator is a consequence of a northwards heat transport across the equator by ocean circulation. Observations suggest that the hemispheric net radiative forcing of climate at the top of the atmosphere is almost perfectly symmetric about the equator, and so the total (atmosphere plus ocean) heat transport across the equator is small (order 0.2 PW northwards). Due to the Atlantic ocean's meridional overturning circulation, however, the ocean carries significantly more heat northwards across the equator (order 0.4 PW) than does the coupled system. There are two primary consequences. First, atmospheric heat transport is southwards across the equator to compensate (0.2 PW southwards), resulting in the ITCZ being displaced north of the equator. Second, the atmosphere, and indeed the ocean, is slightly warmer (by perhaps 2°C) in the northern hemisphere than in the southern hemisphere. This leads to the northern hemisphere emitting slightly more outgoing longwave radiation than the southern hemisphere by virtue of its relative warmth, supporting the small northward heat transport by the coupled system across the equator. To conclude, the coupled nature of the problem is illustrated through study of atmosphere-oceanice simulations in the idealized setting of an aquaplanet, resolving the key processes at work.

Decomposing the meridional heat transport in the climate system

“…It is very interesting to see that right on the Equator, the AHT DSE and AHT LE compensate with each other perfectly, leaving the zero total AHT on the Equator. This situation is different from other model studies, in which the total AHT on the Equator is required to be southward in association with the location of ITCZ at 10°N (Marshall et al 2013). Here it is seen that both the maximum convergence (divergence) of AHT LE and AHT DSE are determined by the ITCZ location.…”

Extratropical forcing and tropical rainfall distribution: energetics framework and ocean Ekman advection

“…[23][24][25] The effect of the AMOC is zonally homogenized by winds in the extratropics, thereby placing the ITCZ in the NH not only in the Atlantic but also in the Pacific. 26 Traditionally, the ITCZ position was thought to be controlled by local mechanisms, such as the shape of coast lines 1,27,28 and Andes topography.…”

Meridional atmospheric heat transport constrained by energetics and mediated by large-scale diffusion

“…An implication is that the peak in zonalmean rainfall resides north of the equator in the annual mean due to hemispheric asymmetry of high-latitude surface heat fluxes which, in turn, reflects northward OHT across the equator due to meridional overturning in the Atlantic Ocean (Figs. 1b,c;Frierson et al 2013;Marshall et al 2014).…”

“…(2). We account for conservation of mass in the meridional overturning circulation energy transport by removing the vertical average MSE, as in Marshall et al (2014), rather than using a barotropic wind correction, as in Trenberth and Stepaniak (2003), because the resulting MOC energy transport has been shown to be more physically relevant on monthly time-scales (Liang et al 2018).…”